As of the end of July 2018, the Let’s Encrypt root, ISRG Root X1, is directly trusted by Microsoft products. Our root is now trusted by all major root programs, including Microsoft, Google, Apple, Mozilla, Oracle, and Blackberry.
Today’s announcement...

Let’s Encrypt is currently providing certificates for more than 115 million websites.

The Certificate Transparency logs which @JuergenAuer has given you already would show you all the certificates ever issued by Let’s Encrypt, but it can take a moment for them to show up. Also everything a week or a month ago.